This invention relates generally to sorbents for removing hydrogen sulfide from hot gas mixtures containing the same and more particularly to metal oxide sorbents for removing hydrogen sulfide from a stream of hot gases produced in gasification of coal.
One of the most promising approaches to producing electrical energy from coal is the integrated gasification combined cycle since it can provide economically and environmentally sound systems which operate with improved thermal efficiency. However, when coal is gasified, sulfur, which is present in most coals, is converted to sulfur-bearing pollutant gases, in particular, hydrogen sulfide. This gas is highly toxic, and its release is unacceptable from the environmental point of view. In addition, it is necessary to protect turbines and related equipment from its corrosive action. Thus, removal of hydrogen sulfide from the coal gasification process stream is essential.
Removal of hydrogen sulfide by previously known low-temperature methods such as scrubbing is not practical because the required low temperatures would impose a severe thermal penalty. For the integrated gasification combined cycle, desulfurization has to be performed at a high temperature typically over 1,000.degree. F. and, in some cases, at a high pressure. A preferred method for removal of hydrogen sulfide would be to contact the gas stream with a sorbent capable of undergoing many cycles of sulfidation and regeneration by means of oxidation. This requires a sorbent which is operational at high temperatures in the presence of reducing gas such as hydrogen and carbon monoxide and which is both mechanically and chemically durable for a prolonged period.
Various metal oxides and related compounds have been tested in the past as sorbents for removal of hydrogen sulfide from a coal gasification stream. In particular, oxides such as iron oxide, copper oxide, and zinc oxide have been used, along with other zinc-based sorbents including zinc ferrite and zinc titanate. While these materials typically show a high initial reactivity with hydrogen sulfide at temperature ranges of interest, they undergo a continuing decrease in reactivity and mechanical deterioration of sorbent pellets due to spalling during repeated cycles of absorption and regeneration, making them undesirable for use in a practical process. Durable sorbent pellets that show a prolonged period of high reactivity with hydrogen sulfide along with resistance to spalling during repeated operating cycles of sulfication and regeneration are therefore needed.